Paying a market rate of interest on reserves could create cash flow problems for a central bank. One problem is that interest on reserves would be paid on a daily basis, but interest earnings would not accrue on a daily basis. Interest on short-term securities is paid on a discount basis rather than on a daily basis and interest on longer term securities is paid at infrequent intervals. Also, inevitably there will be periods in which the yield curve slopes downward, perhaps because the central bank had recently tightened policy and markets expect a return to lower overnight interest rates.
